Deposition is a major problem in the operation of steam generating equipment. Deposition is the accumulation of material on boiler surfaces that can cause overheating and loss of efficiency due to lost heat transfer, as well as circulation restrictions. Such conditions frequently result in unscheduled outages.
Pretreatment of boiler feedwater has undergone significant advances and it is now possible to provide boilers with water closely approaching the ultra-pure level. This degree of purification, however, requires elaborate pretreatment systems. Capital expenditures required to obtain such pretreatment equipment trains can be considerable, and often are not justifiable when balanced against the capability of internal treatment. In addition, conscientious operation and maintenance are also required. The quality of feedwater required is decided by boiler operating pressure, design and heat transfer rates.
The need to provide boilers with high quality feedwater is a natural result of the advances made in boiler performance. The heating surface to evaporation ratio has decreased and is continuing to decrease, resulting in a high heat transfer rate on radiant water wall tubes, often in excess of 200,000 Btu's per square foot per hour and a correspondingly low tolerance for deposition.
Except for supercritical once-through boilers, the majority of boiler systems today can, and do, operate on relatively good quality feedwater, but not approaching the ultra-pure level. Most boiler systems have at least sodium zeolite softening; many have demineralizer trains. The resulting feedwater quality ranges from 0.01 to 2.0 parts per million (ppm) total hardness. But even water of this quality does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary.